Along with widespread use of data processing devices such as copying machines, word processors, computers and so on as well as communication devices, printing apparatuses for these devices that can perform digital image printing with a printhead employing an inkjet printing method are rapidly prevailing. To improve printing speed, these printing apparatuses generally use, for a printing element array where plural printing elements are integrally arranged, a printhead integrally comprising plural ink discharge orifices (nozzles) and plural liquid paths. To perform color printing, these printing apparatuses generally use a printhead comprising plural printing element arrays.
Unlike a monochrome printer which prints texts only, it is necessary for a color printer which prints color images to improve various features, e.g., coloring characteristics, tonality, uniformity and so on, for improved printing image quality. Particularly with respect to uniformity, slight unevenness in nozzle unit which is caused in the printing element array manufacturing process influences the ink discharge amount and discharge direction of each nozzle when printing is performed, ultimately generating unevenness in density of a printed image and causing deterioration in image quality.
In order to reduce such density unevenness, as disclosed in U.S. Pat. No. 4,748,453 (Patent Document 1), a so-called multi-pass printing method has already been proposed. In the multi-pass printing method, pixels of an area that can be printed in one scan are divided into plural groups, and printing of this area is completed by multiple times of scanning. By performing multi-pass printing, the pixels of an area that can be printed in one scan are printed by utilizing different nozzles for each scan. Therefore, the influence inherent to each nozzle which is imposed on the printed image is reduced, and density unevenness in the printed image can be reduced considerably.
However, in a case where the above-described multi-pass printing is performed, a priority color differs depending on the sequence of ink discharged to the printing medium. As a result, one color may be recognized as different colors because of the characteristics of human visual perception.
For instance, assume a case where printing elements provided for respective colors are arranged in order of black, cyan, magenta, and yellow from the right, and scanning is performed by reciprocally moving the printhead in the head arrangement direction (left-and-right direction) (forward scan is the right direction and backward scan is the left direction).
The sequence of printing on a piece of paper corresponds to the aforementioned arrangement order of the printing elements. Therefore, for instance, in a case of printing green (cyan+yellow) pixels of a predetermined area in forward scan, ink is absorbed at the printing position of each pixel on the printing medium in order of cyan and yellow. Therefore, in forward scan, cyan which is absorbed first becomes the priority color, forming a green dot having strong cyan color. Meanwhile, in backward scan, printing is performed while moving the printhead in a direction opposite to the forward scan. Therefore, the sequence of ink discharge is reversed. In backward scan, a green dot having strong yellow color is obtained.
As a result of repeating the above-described scan, green dots having strong cyan color and green dots having strong yellow color are printed depending on whether the green dots are printed in forward scan or backward scan. If the printing medium is conveyed for a distance corresponding to a printing width of the printhead for each of the forward scan and backward scan, the green area having strong cyan color and the green area having strong yellow color are alternately printed for the width of the printhead. As a result, the green area that is supposed to be uniform is considerably deteriorated.
In order to prevent such color unevenness caused by the different ink discharge sequence in the forward scan and backward scan, printing has to be performed by either the forward scan only or the backward scan only. However, the current trend demands for higher speed and higher quality in printing. In such multi-pass printing which realizes printing of respective printing areas in multiple times simply by either the forward scan only or the backward scan only, the time cost required in printing becomes more than doubled. This is not a preferable situation in a printing apparatus.
In view of this, an inkjet printing apparatus disclosed in Japanese Patent Application Laid-Open No. 2001-171151 or No. 2000-079681 is known. In this inkjet printing apparatus, two printing element arrays are provided for each color of ink and arranged symmetrically so that the ink discharge sequence is the same in forward scan and backward scan.
According to this apparatus, color unevenness caused by the different ink discharge sequence in the forward scan and backward scan will not be generated. Furthermore, by utilizing the two printing element arrays provided for each color of ink, it is possible to reduce the number of times of scanning in multi-pass printing and further improve the printing speed.
However, if the number of times of scanning in multi-pass printing is reduced and high-speed printing is performed by the inkjet printing apparatus having two symmetrically arranged printing element arrays for each color of ink, the printing duty per unit time is doubled, resulting a situation where a large amount of ink droplets is discharged before ink is sufficiently fixed to the printing medium.
If a large amount of ink droplets is discharged to a printing medium in a relatively short period of time, the boundary portions of adjacent ink dots join together, causing color mixture between different colors, or bleeding occurs causing blur in texts and ruled lines. Therefore, image quality declines considerably.
By contrast, Japanese Patent Application Laid-Open No. 10-086353 discloses an inkjet printing method which improves ink fixability to a printing medium by providing heaters underneath the platen that is facing the printhead so as to dry the printing medium and ink with the radiation heat. However, an inkjet printing apparatus employing such printing method is compelled to increase its cost largely. Furthermore, the effect of the printing method decreases as the printing speed increases.
Furthermore, Japanese Patent Application Laid-Open No. 58-128862 discloses an inkjet printing method which identifies an image position to be printed in advance and performs printing by overlapping the printing ink with processing liquid for improving printability which contains a component for insolubilizing or coagulating the component in the printing ink. This inkjet printing method improves ink fixability to a printing medium by discharging the processing liquid prior to the printing ink, or discharging the processing liquid on top of the previously discharged printing ink, or discharging printing ink on top of the previously discharged processing liquid and further discharging the processing liquid on top. However, in this method, it is necessary to separately provide a printhead for the processing liquid in addition to the printhead for conventional printing ink. This causes an increased cost and an enlarged apparatus.